In order to obtain high-purity materials, bodies made of this material are remelted. A well-known remelting method is the so-called electro-slag remelting method. In this remelting method, a melting vessel is usually provided in which a melting electrode is remelted. The remelting of the electrode is performed by current flow through the electrode, whereby normally very high currents flow at relatively low voltages. In order to avoid an electrolytic effect that can occur with direct current, alternating current is usually used. In electro-slag remelting, the slag serves as a heating element and refining bath. The current flows via the electrode through the slag zone and the ingot. Due to the resistance of the slag, it heats the tip of the electrode and melts it. The resulting drops of molten metal or metal alloy are refined as they pass through the slag zone. The tip of the electrode is immersed into the slag zone. The slag zone floats on molten metal or metal alloy. The slag zone thus also ensures a sealing-off of the liquid metal from the surrounding atmosphere. Usually, mixtures of calcium fluoride, calcium oxide and aluminium oxide are used as slags. However, pure mixtures of calcium oxide, magnesium oxide, aluminium oxide or silicon dioxide can also be used. The slag must have a low melting point, high stability, low volatility and good reactivity for refining. Impurities in the electrode material are removed both by solution in the slag and by chemical reaction with the slag.
In electro-slag remelting methods, in which the ingot is withdrawn from an opened melting vessel or the melting vessel is moved upwards, it is of decisive importance that the position of the boundary surface between slag and ingot (or between slag and liquid metal) relative to the mould is kept almost constant during the process, among other things to avoid thermal overload or damage to the plant. The challenge here is to compensate the volume build-up on the ingot, which is determined by the melting rate, correspondingly by the withdrawal speed of the ingot or by the lifting speed of the mould. Due to the different (partly indefinite) density of the melt material in the area of the electrode and the ingot, simple volume calculations from the melt rate show an insufficient accuracy, which can ultimately lead to drift effects. Reliable determination and control of the positions are of decisive importance for process and operational safety. Otherwise, there is a risk of slag leaking (if the position is too deep in the melting vessel) or slag overflow (if the position is too high in the melting vessel).
The electro-slag remelting method differs significantly from other processes, such as continuous casting, in that in electro-slag remelting there are not only two boundary surfaces between ingot and metal melt and between melt and gas phase, but three boundary surfaces: ingot/metal melt, metal melt/slag and slag/gas phase. It is not sufficient for the process control to determine the position of one of the three boundary surfaces, since the height of the slag zone is not constant. Instead, slag is continuously discharged from the slag zone, forming a slag skin on the ingot produced. In addition, the height of the molten metal is not constant. Thus, the determination of the position and height of the slag zone during electro-slag remelting is extremely complex and, in particular, much more complex than in methods with only two boundary surfaces.
In order to overcome the problem, various solutions are applied which substantially rely on monitoring the slag level during the remelting method. Both optical measuring methods and radar-based distance measurements are employed. EP 2 386 366 A2 discloses a radar-based measurement of the slag level. The distance measurement used in earlier times using isotope radiations is no longer up to date.
Besides the difficulties resulting from the volumetric calculation, the determination of the position is further complicated by the fact that a layer of slag solidified at the ingot is permanently withdrawn from the system and as a consequence the mass (or height) of the slag can change during the course of the process. With prior art methods, only the position of the surface of the slag zone can be directly determined, but not its extent, i.e. its height in the melting vessel.
A visual check of the slag level is not easily possible during the method, since normally closed systems are used, i.e. the melting vessel is closed with a hood. A continuous visual monitoring of the slag level would not be practicable anyway, because there is a strong smoke development so that abrupt changes of the slag level would not be noticed immediately. Closed systems generate more dust due to the reduced gas exchange. Video systems, which were sometimes used in the prior art, tended to take incorrect measurements due to dust and smoke generation; moreover, the camera's viewing angle and the temperatures on the inner wall do not permit a clear determination. Furthermore, it can be useful to allow variation of the slag zone within certain limits in order to increase the service life of the mould. This requires a particularly careful and continuous determination of the slag level and can be determined from the heat flow over the vessel wall.
JP S63-72837 A describes a method for electro-slag remelting in which the temperature is measured in a mould wall over the height of the mould. From this, conclusions are to be drawn about the height and position of the slag zone. However, this method has proved to be impracticable in practice, since by using only one measuring group only one temperature measurement is performed and no heat balance can be established. In contrast to a mere temperature measurement, however, the heat balance can be used to determine the localization of the slag zone much more accurately and correctly. For this, at least two measuring groups are necessary, as it is intended here according to invention. FIG. 3 in JP S63-72837 A also shows in an illustrating manner that the thickness of the slag skin has a considerable influence on the temperature measurement.